California's governor, Arnold Schwarzenegger, warned on Thursday that his state "is headed toward one of the worst water crises in its history".

Now new research suggests that the three-year drought in the Golden State may be a consequence of the expanding tropics, which are gradually growing as human emissions of greenhouse gases warm the planet.

Climate scientists have documented a slow progression of low-latitude weather systems towards the poles, and this has been matched by rising temperatures in many temperate regions. Deciding whether this broadening of the tropical belt is linked to the greenhouse effect has been difficult, however.

Part of the reason, explains Thomas Reichler of the University of Utah, is that there are many ways of defining the tropics. Geographically, the tropical belt is contained between the Tropics of Cancer and Capricorn. It is also the region on either side of the equator where temperatures tend to be hot and humid all year.

Where weather forms

But the simplest and most easily tracked characteristic of the tropics lies high above, at the boundary between the troposphere, where weather systems form, and the stratosphere above it.

Over the tropics, the tropopause, as this boundary is known, tends to lie several kilometres higher up in the atmosphere. The change in altitude is relatively easy to measure. "It is much more difficult to detect significant changes in the lower levels of the atmosphere and surface rainfall pattern," says Jian Lu of the US National Center for Atmospheric Research in Colorado.
Creeping outwards

Using the tropopause, Lu and Reichler tracked the position of the tropical belt since the 1960s and found it has slowly been getting wider. "There is a lot of natural variation from year to year," says Reichler, "but we see a slow, gradual change." On average, the tropical boundaries are moving 0.7 degrees towards the poles each decade. This amounts to roughly 70 kilometres per decade, or 350 kilometres in 50 years.

The team then plugged their data into a leading climate model. If the model included human emissions, it matched the real data. Without the emissions, it didn't.

"Our main conclusion is that greenhouse gases and [the depletion of stratospheric] ozone are the culprits for the widening," says Lu. "These two work in the same direction, both pushing the boundary of the tropics polewards."

Subtropical deserts

Reichler says that the expansion of the subtropics is more feared than the widening of the tropical zone itself. While the tropical belt is hot and humid, the subtropics suffer from severe drought. The Sahara and Sahel are both subtropical regions.

Southern California is already subtropical in the summer. But with climate change, dry conditions could spread to areas like northern California, Washington and Utah, which now get far more rain and snow.

According to a recent survey by the California Department of Water Resources, the snowpack on California's mountains is currently carrying only 61% of the water of normal years. The Sierra snowpack alone provides two-thirds of California's water supply, and these mountains have so far only received one-third of the expected annual snowfall, despite December and January normally being the wettest months.